How Does Thermonuclear Fusion Work?

(Transcript of the video commentary.)

Using the energy of atomic nuclei is certainly one of the most promising ways to produce a sufficient amount of electricity and fully satisfy humankind’s energy needs. So far, we have mastered the simpler fission of heavy nuclei but for several decades, research and experimental work has been going on to master the fusion of the nuclei of light elements which could provide us with up to 10 times more energy than the fission of uranium nuclei. Imitating thermonuclear fusion, which powers the entire universe and our Sun, will not be easy at all under Earth conditions. But experts are gradually managing to solve all the technological challenges and related problems and in the near future, the efforts will be crowned with a functional thermonuclear power reactor.

Fusion of light nuclei is a clean source of energy with virtually unlimited fuel supplies, no hazardous waste production and minimal environmental impact. And how does this fusion take place? The basis is protons and neutrons which hold together the strong nuclear forces in the tiny atomic nucleus. If we would like to overcome this and separate these particles from the nucleus, we have to supply the nucleus with a certain amount of energy called binding energy. This energy is directly proportional to the difference between the total mass of all nucleons in the nucleus and the true mass of the nucleus at rest.

An even more interesting parameter is the binding energy per nucleon. The light elements at the beginning of the periodic table and the heavy elements at the end have a lower binding energy per nucleon value than the nuclei of the stable elements in the middle of the table. When certain conditions are met, this can be used energetically either by synthesizing a light nuclei or by the fission of a heavy nuclei. In both cases, the binding energy per nucleon increases for the new nuclei and the reactions release energy related to the increased stability of the nuclei.

In order for nuclei to join during fusion, it is necessary to get over the electrostatic repulsive forces between the positively charged nuclei and bring them closer to a distance where strong nuclear forces begin to act. The repulsive forces can be overcome dynamically, when the nuclei are given sufficient velocity and they move together by inertia to the fusion distance or statically, when an external force impacts the nuclei that overcomes the Coulombic repulsion and the nuclei fuse. A synonym for the speed of particles is their temperature. A high temperature at the level of several tens to hundreds of millions of kelvin means a sufficiently high speed of the particles. And an external force overcoming the repulsive forces of the nuclei can be thought of as compressing them into a very small volume. Heating and compression must be done simultaneously in nuclear fusion.

So far, the only functioning fusion reactor in our solar system is the Sun. Every second it burns 500 million tons of fuel in its core which are protons or nuclei of ordinary hydrogen. Under Earth conditions, this relatively inefficient type of fusion cannot be energetically used, as both the probability of these reactions and the energy gain are very low. But the fuel can be other light elements and their isotopes. Choosing the right combination for a fusion reaction depends only on the achievement of the required ignition temperature, the willingness of the nuclei to fuse and the energy gain of the given reaction. The more energy released during the reaction, the more energetically interesting the reaction is, of course.

The most promising thermonuclear fusion reaction that humankind would like to try to ignite and use is the fusion of deuterium and tritium into a helium nucleus and a free neutron, taking place at a temperature of 160 million Kelvin. At this temperature, matter is in a state of ionized gas known as plasma.

Deuterium is a naturally occurring stable isotope of hydrogen containing one proton and one neutron. Although there are about six and a half thousand atoms of ordinary hydrogen per atom of deuterium, deuterium can be obtained relatively easily from seawater. The second fuel isotope is slightly radioactive tritium, which is also an isotope of hydrogen that contains two neutrons in its nucleus. It occurs naturally on the Earth very rarely but is artificially created as a by-product in existing nuclear reactors.

During the fusion reaction of deuterium and tritium nuclei, apart from the helium nucleus, one more high-energy neutron is produced. This fact will be reflected in the fusion reactor equipment, where the reaction will take place, by the embrittlement of the materials of the reaction chamber as a result of the high flow of neutrons and the associated activation.

The production of electricity in fusion power plants will only be possible if we get significantly more energy from the fusion reaction than we use to ignite and sustain it. The ratio of the energy produced to the energy required to sustain the fusion reaction is called the gain factor. The conditions necessary for a fusion reactor to achieve the required output were already formulated in the middle of the last century by John David Lawson.

The first parameter of Lawson’s criterion is temperature. The higher the temperature, the more energy the particles gain and the more likely they are to collide and fuse. The second parameter is the density of the fuel. In a denser fuel, the particles are more likely to meet and fusion will occur. The third and last parameter of Lawson’s criterion is time or the confinement time of particles with the desired temperature and density in the reaction space. Mathematically, the Lawson criterion could be defined as the product of density and time, which is a function of residual temperature.

From the formulation and results of Lawson’s criteria, there are essentially two options for achieving an economically viable fusion. The first possibility is to maintain a relatively low-density plasma for a longer period of time, which is the basic principle of magnetic confinement. The second option is the opposite and consists in keeping the plasma for a shorter time but with a significantly higher density. This is used in inertial confinement.

The conditions for the successful development of fusion reactions are the achievement of the necessary product of the plasma density and confinement time and the achievement of a specific ignition temperature at which the given reaction can take place. These conditions result in two basic types of confinement: magnetic and inertial.

In magnetic confinement, the plasma is enclosed in a reaction space, formed by a magnetic field, where charged particles move along closed field lines and are heated to the fusion temperature. The magnetic field is chosen in this case because no solid material could withstand such high temperatures. Relatively lower plasma density is characteristic for magnetic confinement. On the other hand, the confinement time ranges from tens to hundreds of seconds.

Tokamaks and stellarators are based on the idea of magnetic confinement. Tokamaks create an additional toroidal magnetic field excited by an electric current flowing directly through the plasma. The most famous tokamak is the international thermonuclear experimental reactor ITER, which is being built in Cadarache, France.

Stellarators, unlike tokamaks, create a suitably shaped magnetic field by adding external helical magnetic coils, usually of a very bizarre shape. Stellarators could work continuously in the future whereas tokamaks are more pulse facilities.

During inertial confinement, it is about achieving a short-term high plasma density by compressing a pellet of thermonuclear fuel using a beam of radiation, most often generated by a powerful laser. After evenly compressing the fuel pellet from all sides, the density and temperature of nuclear fusion is reached in its centre. But the compression and heating is so rapid that the fusion ignition occurs before the fuel is dispersed. The very high density allows a significantly shorter confinement time according to the Lawson criterion, which can be in the order of nanoseconds.